if you’re into planes or WW2 history then I can recommend a visit to the Dehavilland museum just off the M25. spent 4 hours there and ran out of time checking things out
Must visit that.
Also worth a drop in is the Gatwick Aviation Museum, mainly 50’s and 60’s military aircraft but also some interesting exhibits related to the history of Gatwick Airport.
I grew up in St.Albans and vaguely remember the place being mentioned but it definitely looks worthwhile visiting - nice one.